The present invention relates generally to controlling the output of lights. More particularly, embodiments of the invention relate to a method and apparatus that use an LED as a light sensor for detecting light levels in an area or room.
Lighting control circuits are used with electronic dimming ballasts. These ballasts control the output of lights, such as fluorescent lights, that illuminate areas such as rooms, offices, patios, etc.
Traditionally, photocells and photodiodes are used as photo-transducers or light sensors for lighting control systems. A photocell is a device that detects light in a controlled area or room. It then uses information from the light, e.g., illumination level, to adjust light output in the controlled area.
Photocells and photodiodes are wide spectrum sensors and they respond to a spectrum much wider than the spectrum perceived by the human eye. This is acceptable for a variety of lighting control systems including systems operating in areas were the controlled light has the same spectrum all times, e.g., where only fluorescent lights are delivering the illumination. If the spectrum distribution remains the same, the resultant electrical energy is proportional to visible energy or light. Hence, a lighting control system can be adjusted to keep the visible light level constant.
Typically, the light in a controlled area or room has two or more different contributing light sources, e.g., artificial light plus sunlight. This is the condition commonly encountered in real life. For example, the controlled light source is typically fluorescent lights and the variable or xe2x80x9cdisturbingxe2x80x9d source is the sun, i.e., daylight. Note that for the purposes of discussion, the terms sunlight, daylight and natural light are used synonymously. Similarly, the terms electrically produced light and artificial light are used synonymously. Artificial light would include for example fluorescent light, incandescent light, etc.
The radiometric energy spectrum of sunlight is wider than that of electronically produced light such as fluorescent light. Thus, different light sources could have different energy spectrums. Also, the human eye perceives only a part of the energy spectrum emitted by all available light sources, e.g., sun light, incandescent light, fluorescent light, etc. Research done on a variety of human subjects shows that the sensitivity of the human eye varies with the lighting level. It is widely accepted by specialists in the field that under daylight conditions the spectral response of the human eye can be approximated by the so-called xe2x80x9cphotopic curve.xe2x80x9d This has a well-known bell shape and ranges from about 460 nm to 680 nm wavelengths, with the peak in the region of 560 nm. Some research has shown that under poor illumination conditions the human eye changes its spectral sensitivity. A new characteristic has been devised for this behavior. It is called the xe2x80x9cscotopic curve.xe2x80x9d This is centered at about 410 nm and covers the spectrum from about 380 nm to 450 nm. In analyzing its overall behavior, it is perhaps appropriate to say loosely that the human eye can perceive light in the range of 400 nm to 700 nm.
A problem arises because most conventional photo-transducers capture or detect the entire energy spectrum produced by all light sources. Thus, when the photo-transducer transforms the captured light energy into a current, it does not distinguish between different wavelengths of light, i.e., sunlight and artificial light. This conventional design of lighting control systems is based on the assumption that the current represents visible light. Unfortunately, this is a poor assumption. In one known light controller circuit, for example, a current resulting from both natural and artificial light components is interpreted by a subsequent circuit as though it is a current merely resulting from the artificial light contribution. Accordingly, the system dims the artificial lights until the resultant voltage equals a set point or preset illumination level. This is problematic because the resultant voltage is derived from both natural and artificial light components which include non-visible energy, while the preset illumination level is set according to visible light standards, e.g., 40 foot candles. Consequently, in most cases, this results in full dimming of the artificial lights while the incoming daylight clearly provides insufficient illumination for a typical room.
Some circuits use a light filter to allow only the visible spectrum to reach the photo-transducer. For example, an optical filter placed over a photo-transducer can achieve this. This would mimic the photopic curve or visible spectrum. Light sensors using optical filters are much more efficient than conventional photocells used without such filters. Optical filters, however, are expensive. These special pick-up heads are typically used in some professional applications. Note, as used herein, the term optical sensor is used to mean a photo-transducer used with an optical filter.
Thus, it is desirable to have an alternative lighting control circuit that can detect a spectrum of light close to that which the human eye detects.
The present invention achieves the above needs with a new lighting control circuit. More particularly, the present invention provides a lighting control circuit having an LED that outputs a first signal in response to being exposed to radiation, a detection circuit coupled to the LED. The detection circuit is configured to generate a second signal from the first signal. A driver circuit is coupled to the detection circuit, and the driver circuit is configured to generate a third signal to control an illumination level of one or more lights. The third signal is varied in response to the second signal.
In another embodiment, the driver circuit receives the second signal and compares it to a fourth signal. The driver circuit is configured to match the second signal with the fourth signal via a loop, thereby either raising or lowering the illumination level of one or more lights until the second signal and the fourth signal match.
In another embodiment, the first signal is amplified. In another embodiment, a light spectrum detected by the LED substantially mimics the photopic curve. In yet another embodiment, the fourth signal is adjustable and represents a desired illumination level. In yet another embodiment, the lighting control circuit adjusts the ambient light in response to changes in the ambient light.
In another embodiment, a lighting control circuit includes an LED that outputs a first signal in response to being exposed to radiation. A detection circuit couples to the LED and is configured to generate a second signal from the first signal. A driver circuit couples to the detection circuit and is configured to generate a third signal to control an illumination level of one or more lights. The third signal is varied in response to the second signal, and the driver circuit receives the second signal and compares it to a fourth signal. Also included is a loop which has an opto-electric path and an electronic path. The opto-electric path travels from a light source controlled by the lighting control circuit to the LED via the radiation from the light. The electronic path travels from the LED to the light source via the lighting control circuit. The driver circuit is configured to match the second signal to the fourth signal via the loop, thereby either raising or lowering the illumination level of one or more lights until the second signal and the fourth signal match.
In another embodiment, a method for controlling the brightness level of a light is provided. The method includes exposing an LED to radiation, outputting from the LED a first signal in response to the radiation exposure, generating a second signal from the first signal, and generating a third signal to control an illumination level of one or more lights, wherein the third signal is varied in response to the second signal.
In another embodiment, the step of generating the second signal includes amplifying the first signal. In yet another embodiment, the step of generating the third signal includes comparing the second signal to a fourth signal and matching the second and fourth signals. In yet another embodiment, the step of matching further included adjusting the ambient light level until the second signal matches the fourth signal.
In another embodiment, a lighting control circuit includes an LED that emits light when driven by a current and detects light when the current is turned off. The LED outputs a first signal in response to a detected light. A driver circuit couples to the LED and provides a current-to-voltage transfer ratio to operate with the LED. A multiplexer couples to the driver circuit and selects a first mode and a second mode, the LED having a first polarity during the first mode and a second polarity during the second mode. During the first mode the LED emits light when driven by a current. During the second mode the LED detects light and generates the first signal when the current is turned off. The lighting control circuit controls an illumination level of one or more lights in response to the first signal. In another embodiment, the LED detects a spectrum that approximates a photopic luminosity curve. In yet another embodiment, the photopic luminosity curve approximates a C.I.E. relative photopic luminosity curve.
Embodiments of the present invention achieve their purposes in the context of known circuit technology and known techniques in the electronic arts. Further understanding, however, of the nature, objects, features, aspects and embodiments of the present invention is realized by reference to the latter portions of the specification, accompanying drawings, and appended claims. Other objects, features, aspects and embodiments of the present invention will become apparent upon consideration of the following detailed description, accompanying drawings, and appended claims.